The trend of a semiconductor device in recent years has been a highly integrated structure, which entails finer interconnects of a circuit and a smaller distance between the interconnects. In fabrication of the semiconductor device, many types of materials are deposited in a shape of film on a silicon wafer repeatedly to form a multilayer structure. It is important for forming the multilayer structure to planarize a surface of a wafer. A polishing apparatus for performing chemical mechanical polishing (CMP) is typically used as one technique of planarizing the surface of the wafer (also called a chemical mechanical polishing apparatus).
This chemical mechanical polishing (CMP) apparatus typically includes a polishing table supporting a polishing pad thereon for polishing a polishing target (such as a wafer) and a top ring for holding a wafer for holding and pressing the polishing target against the polishing pad. The polishing table and the top ring are respectively driven to rotate by a drive section (e.g., motor). The polishing apparatus is further provided with a nozzle for supplying a polishing liquid onto the polishing pad. When polishing a wafer, the top ring presses the wafer against the polishing pad, while the polishing liquid is supplied onto the polishing pad from the nozzle. In this state, the top ring and the polishing table are moved relative to each other, whereby the wafer is polished to have a planarized surface. Examples of a scheme for holding the top ring and the drive section of the top ring include a scheme of holding the top ring and the drive section of the top ring at an end of a swing arm (cantilever arm) and a scheme of holding the top ring and the drive section of the top ring to a carousel.
If the polishing of the polishing target is not sufficient, insulation between circuits may not be secured and a short-circuit may result. On the other hand, excessive polishing may result in problems such as an increase of resistance values due to a decrease in the cross-sectional area of interconnects or the interconnects themselves being completely removed and circuits themselves being not formed. Therefore, the polishing apparatus is required to detect an optimum polishing end point.
A method of detecting a change in a polishing frictional force when polishing reaches a point where there is a change from a substance to another substance of a different material is known as one of polishing end point detection means. A semiconductor wafer which is a polishing target has a multilayer structure made of different materials such as a semiconductor, conductor and insulator, and a frictional coefficient varies among layers of different materials. For this reason, this method detects a change in the polishing frictional force when the polishing reaches a point where there is a change from a substance to another substance of a different material. According to this method, a time point at which polishing reaches the different material layer is an end point of polishing.
The polishing apparatus can also detect a polishing end point by detecting a change in the polishing frictional force when the polishing surface of the polishing target is changed from a non-flat state to a flat state.
Here, the polishing frictional force generated when the polishing target is polished appears as a drive load of the drive section that drives the polishing table or the top ring to rotate. For example, in a case where the drive section is an electric motor, the drive load (torque) can be measured as a current that flows through the motor. For this reason, it is possible to detect a motor current (torque current) using a current sensor and detect an end point of polishing based on a change in the detected motor current.
Japanese Patent Application Laid-Open No. 2004-249458 discloses a method of measuring a polishing frictional force using a motor current of a motor that drives a polishing table and detecting an end point of polishing in a scheme in which a top ring is held to an end of a swing arm. In a scheme in which a plurality of top rings are held to a carousel, an end point detection method is available which detects a torque current (motor current) of a carousel rotation motor (Japanese Patent Application Laid-Open No. 2001-252866, U.S. Pat. No. 6,293,845). A scheme is also available in which a top ring is driven in a lateral direction by a linear motor attached to a carousel. According to this scheme, an end point detection method through detection of a torque current (motor current) of a linear motor is disclosed (U.S. Patent Application Publication No. 2014/0020830).